ORGANS OF THE SPAT 65 



accounts I judge that the fixative was an organic secretion of different 

 origin from both larval and spat shells. 



There can be little doubt that the larval and spat shells are built by 

 the apposition of new matter at the growing edges, and that this matter is 

 secreted from glands situated in the thickened margins of the mantle. 

 Whether they are secreted from the same set of glands in both cases is 

 more than doubtful. Thin margins of the protruding mantle, as well as 

 prepared sections show more or less elongated unicellular glands, the 

 larger of which have a narrow neck and an irregular broader fundus, con- 

 taining a refringent semi-fluid material. The fixative material represents 

 such a quantity of matter that it could scarcely be produced quickly enough 

 by the same means as that of the shell. Besides there would be an insur- 

 mountable difficulty in the bringing of the mantle margins far enough 

 outside of the shell to be applied to the proper spot. Fixation is no hap- 

 hazard process. Such a sudden change in the mode of existence of the 

 organism, occurring with regularity at a definite period in the life of each 

 individual, can not have failed to call out a correlative organization, per- 

 mitting accommodation to a new method of living. 



Turning to the oldest larval stages, immediately preceding fixation, I 

 find a gland, the byssus gland, capable of supplying secreted matter, that 

 might be used in fixation, and, at the same time, situated in an organ, the 

 foot, capable of bringing the mouth of the gland in the heel to the proper 

 spot on the outside of the shell. The usefulness of the foot as an organ of loco- 

 motion, as a clinging organ, as an organ of fixation, had appealed to me 

 for some time, but I had no direct evidence to support the view that it was 

 really the organ of final attachment. Upon observing the soldering matter 

 on the outside of the left valve of the lately fixed spat, however, the con- 

 clusion became irresistible. Re-examining the byssus-gland I find that 

 it occupies in full grown larvae a considerable portion of the inside of the 

 foot (Plate VI, figs. 4, 5), and consists of a median and two lateral lobes, 

 with a main duct running through the median portion and continuing to 

 the external opening at the end of the heel. The cells are large and dis- 

 tended with a transparent secretion, which presses the small nuclei towards 

 the wails. In the youngest spat, on the other hand, the byssus-gland is 

 relatively inconspicuous and its cells shrunken and collapsed. 



As to the method of applying the cementing substance, the presence 

 of a little hook or knob of secretion (Plate VI, figs. 11, 16), near the upper 

 anterior edge of the shell, gives a hint that, while the larva is lying upon its 

 left valve, the foot is thrust forwards and upwards until the point of the 

 heel, on which opens the duct of the byssus-gland, comes to this spot, and 

 then the secretion is poured out, flowing between the shell and the sub- 

 stratum, until fully discharged, when the withdrawal of the foot, breaking 

 the current of the outflow, leaves the detached end of the hardening se- 



